The invention relates to an interrogating station for interrogation and identification--using high-frequency signals--of an object which is moved along an interrogation path such as a conveyor, said station being provided with a passive transponder fixed to the object for transmitting its own identification code in response to an interrogation signal, and an antenna system operable for transmitting the interrogation signal and for receiving the identification code respectively. Such an interrogating station is known in practice.
With such a station objects on which or in which a transponder is fixed are identified using the identification code belonging to the transponder. For this, a high-frequency carrier signal is transmitted in the interrogation path and received by the transponder. The transponder is a so-called passive transponder which is excited or charged by the above-mentioned carrier signal. In reaction to this, the transponder transmits the above-mentioned identification code, which is unique for each transponder. The design is generally such that during a particular interrogation period one or more successive transponders are excited or charged, following which the identification code is transmitted in the succeeding receiving period by the transponders.
The antenna system used in such an interrogating station is disposed near the interrogation path, such as a conveyor, along which the objects, such as pieces of luggage, are being moved. The antenna system used for the interrogation and reading or receiving of the identification code is disposed near or on the conveyor. It will be clear that the use of relatively small single antennae gives inadequate cover of the interrogation path, so that dead angles or zones in which no reading takes place occur.
In practice, attempts have been made to remedy this by e.g. fitting a frame antenna of rectangular or square shape around the conveyor. The object is, of course, to obtain a reading in every point within the interrogation path for all possible transponder orientations. Such a frame antenna is then fitted at right angles to the conveyor.
Experiments with objects with transponders fitted to them moved along the conveyor in all orientations relative to a frame antenna have shown:
that for horizontal transponders with their axis parallel to the central axis of the antenna the interrogation/reading behaviour was good; and
that for transponders in a vertical plane with the transponder axis horizontal or vertical the interrogation/reading behaviour was good except in a clear dead zone around the central axis of the frame antenna. No reading takes place in this dead zone. This is due to the fact that the elongated antenna of the transponder is sensitive essentially in one direction.
The object of the invention is to eliminate this problem.